Broadly speaking, metal-clad laminates (for printed wiring board substrates) are known. As the electronics industry progresses, such laminates must accommodate increasingly smaller circuitry that is operating at increasingly higher speeds. Generally speaking, a shorter time delay (of signal transmission rate) is possible by making the insulating layer thinner, while also reducing its dielectric constant. A dielectric having a low dielectric constant, e.g., having a dielectric constant lower than 4.3, 4.1, 4.0, 3.7, 3.5, 3.2, 2.0, 2.9, 2.8, 2.7, 2.6, 2.5, 2.4, 2.2, 2.0, 1.9, 1.8, 1.7, 1.6, or 1.5, will hereafter be referred to as a “low-k dielectric.”
A low-k dielectric can sometimes fail due to arc-tracking. The low-k dielectric separates two conductive paths, and any electric arc (over the dielectric) will typically cause a short circuit. Such arc tracking can cause catastrophic failure.
Japanese Patent Application Laid-Open No. 8-250860 discloses a metal foil-clad laminate obtained by applying an aromatic polyamic acid solution onto a metal foil and then heat-treating the applied solution to imidize the aromatic polyamic acid. However, the process may become increasingly costly (and prone to quality errors), particularly as circuitry designs become more advanced.
Generally speaking, polyimides obtained from aliphatic (chain) tetracarboxylic dianhydride can have an advantageously low dielectric constant, but such polyimides also tend to have poor heat resistance (for processes such as welding), thereby greatly reducing their practical utility. Although polyimides obtained from alicyclic tetracarboxylic anhydrides have improved heat resistance (as compared to those obtained from the aliphatic chain tetracarboxylic dianhydride), such polyimides tend to have low solubility to solvents and can therefore be difficult to coat onto another substrate, such as metal. Although polyimides derived from compositions, such as, 1,2,4,5-cyclopentanetetracarboxylic dianhydride and bicyclo[2.2.2]octa-7-ene-2,3,5,6-tetracarboxylic dianhydride, can have adequate solvent solubility, the resulting laminate can have unacceptably poor flexibility.